Polyolefinic materials encompass a variety of polymers ranging from semi-rigid polypropylene (PP) to soft ethylene polymers. They can be used to produce a variety of foam products. Most polyolefin foams are closed-cell foams, which are buoyant, resilient, tough, flexible, and resistant to chemicals and abrasion. Therefore, polyolefin foams are useful for packaging, construction, insulation, sports, leisure and footwear applications.
Copolymers of ethylene and vinyl acetate (EVA) have been widely used as base resin polymers in foam applications for many years. Crosslinked EVA foams, expanded with chemical blowing agents, provide an attractive balance of resilience, durability and other physical properties required for soling applications in footwear. These properties are provided at low density, which is desirable for lighter weight shoes, and at an attractive cost. EVA may present limitations in attaining a balance of softness (e.g., surface softness), low compression set, and high resilience. Also, as foam processes move more toward one-step injection molding, achieving balanced properties using EVA foam may become difficult.
Foams made from ethylene acrylate copolymers (also referred to as ethylene-acrylic acid ester copolymers), such as ethylene-methyl acrylate copolymer (E/MA) with high MA content, are generally soft, have low density and are highly resilient.
E/MA foam may be weak in mechanical properties, such as tear strength and tensile strength, and may be difficult to crosslink.
There is a continued need to develop new products to expand the performance window of known polyolefin foams, such as the foam footwear market, to reduce costs, and to improve manufacturing process. It is also desirable to improve the crosslinking and mechanical properties while retaining the inherent merits of E/MA foams.